The biosynthesis of proteins in genetically altered procaryotic cells results in expression of a protein having an N-formyl-methionine attached at the amino terminus. Since the addition of N-formylmethionine to the native protein may alter its biological activity, conformational stability, antigenicity, etc., it is most desirable, if possible, to remove it.
By insertion of a cleavage site between the amino-terminal methionine and the desired native peptide, one in theory has a greater degree of flexibility in the methodology selected for achieving production of the desired native peptide. In fact, however, there are only a very limited number of practical methods for achieving selective cleavage.
For example, in those native proteins in which methionine is not present, cyanogen bromide mediated cleavage (methionine being the selective cleavage site) has proven to be a very effective method for generating native protein. In fact, however, the absence of methionine is a rare occurrence in moderately-sized peptides and proteins. Consequently, a most important objectie is to discover a method by which the aminoterminal methionine can be cleaved with generation of the native biosynthetically-produced protein.
A key and unexpected observation determining any approach for generating native protein from amino terminal methionyl protein is that the .alpha.-amino group of the biosynthetic expression product, for example, N.sup..alpha. -methionyl-(human growth hormone), is not formylated. This fact was first observed through chemical modification of the expression product with cyanate and later was confirmed by automated Edman sequence analysis. The E. coli expression organism, although incapable of cleavage of the initiating methionine, nevertheless, did effect removal of the N.sup..alpha. -formyl group, presumably enzymatically. Consequently, the .alpha.-amino group of the amino-terminal methionine expression product is directly available (potentially at least) for cleavage.
Having a free .alpha.-amino group on the amino-terminal methionyl expression product gives rise to the possibility of other approaches for methionyl removal. One approach, a selective Edman-type amino terminal sequential cleavage using phenyl isothiocyanate, for practical application requires the absence of lysine residues in the native protein. However, only the rarest of proteins will be free of lysine.
Another possible approach is the use of an exopeptidase; in principle, these will achieve stepwise amino acid scission. Reports of successful utilization of exopeptidases in synthetic studies have been quite scarce for a number of reasons, including their heterogeneous nature, expense, susceptibility to denaturation, and, most importantly, their wide variability in specific and non-specific action against differing substrates.
A new methodology has been discovered which represents novel chemical procedures and provides new compounds useful in achieving selective amino-terminal cleavage to produce the desired end peptide product. It is to such a process and to compounds useful therein that this invention is directed.